Methods of forming electrochemical cells

ABSTRACT

Methods of forming electrochemical cells are described. In some embodiments, the method can include providing an electrochemical cell having an electrode with at least about 20% to about 99% by weight of silicon. The method can include providing a formation charge current at greater than about 1 C to the electrochemical cell. Alternatively or additionally, the method can include providing a formation charge current at a substantially constant charge voltage to the electrochemical cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/596,069, filed Dec. 7, 2017. The entirety of the above referencedapplication is hereby incorporated by reference.

BACKGROUND Field of the Invention

The present disclosure relates to electrodes, electrochemical cells, andmethods of forming electrodes and electrochemical cells. In particular,the present disclosure relates to methods of formation ofelectrochemical cells, including lithium-ion batteries.

Description of the Related Art

A lithium-ion battery typically includes a separator and/or electrolytebetween an anode and a cathode. In one class of batteries, theseparator, cathode and anode materials are individually formed intosheets or films. Sheets of the cathode, separator and anode aresubsequently stacked or rolled with the separator separating the cathodeand anode (e.g., electrodes) to form the battery. For the cathode,separator and anode to be rolled, each sheet must be sufficientlydeformable or flexible to be rolled without failures, such as cracks,brakes, mechanical failures, etc. Typical electrodes includeelectro-chemically active material layers on electrically conductivemetals (e.g., aluminum and copper). For example, carbon can be depositedonto a current collector along with an inactive binder material. Carbonis often used because it has excellent electrochemical properties and isalso electrically conductive. Electrodes can be rolled or cut intopieces which are then layered into stacks. The stacks are of alternatingelectro-chemically active materials with the separator between them.

SUMMARY

In certain embodiments, a method of forming an electrochemical cell isprovided. The method can include providing an electrochemical cellhaving a rated capacity at 1 C, and providing a formation charge currentat greater than about 1 C to the cell. The electrochemical cell caninclude a first electrode and a second electrode. At least the firstelectrode can include at least about 20% to about 99% by weight ofsilicon. The electrochemical cell can also include a separator betweenthe first electrode and the second electrode. The electrochemical cellcan include an electrolyte in contact with the first electrode, thesecond electrode, and the separator.

In some embodiments, the formation charge current can be provided atgreater than about 2 C. In some embodiments, the formation chargecurrent can be provided at less than or equal to about 20 C. In someexamples, the formation charge current can be provided for about 3minutes to about 60 minutes. In some instances, providing the formationcharge current can comprise charging to partial formation. For example,charging to partial formation can comprise charging to about 10% toabout 90% of total capacity of the electrochemical cell, to about 20% toabout 70% of the total capacity of the electrochemical cell, or to about30% to about 50% of the total capacity of the electrochemical cell.

In some embodiments, providing the formation charge current can compriseproviding a substantially constant formation charge current. In someembodiments, providing the formation charge current can compriseproviding the formation charge current at a charge voltage from about 3volts to about 6 volts. The electrochemical cell can be a lithium-ionbattery and providing the formation charge current can compriseproviding the formation charge current with substantially no lithiumplating.

In some embodiments, the first electrode can be an anode. The firstelectrode can include a silicon-dominant electrode. For example, thefirst electrode can comprise the silicon at about 30% to about 99% byweight, at about 40% to about 99% by weight, at about 50% to about 99%by weight, at about 60% to about 99% by weight, at about 70% to about99% by weight, or at about 80% to about 99% by weight. In someinstances, the first electrode can further comprise graphite.

In certain embodiments, a method of forming an electrochemical cell isprovided. The method can include providing an electrochemical cellhaving a rated capacity at 1 C, and providing an initial formationcharge current at a substantially constant charge voltage to theelectrochemical cell. The electrochemical cell can include a firstelectrode and a second electrode. At least the first electrode caninclude at least about 20% to about 99% by weight of silicon. Theelectrochemical cell can also include a separator between the firstelectrode and the second electrode. The electrochemical cell can includean electrolyte in contact with the first electrode, the secondelectrode, and the separator.

In some embodiments, the charge voltage can be from about 3 volts toabout 6 volts. The initial formation charge current can be provided forabout 3 minutes to about 60 minutes. Providing the initial formationcharge current can comprise charging to partial formation. For example,charging to partial formation can comprise charging to about 10% toabout 90% of total capacity of the electrochemical cell, to about 20% toabout 70% of the total capacity of the electrochemical cell, or to about30% to about 50% of the total capacity of the electrochemical cell.

In some embodiments, the initial formation charge current can beprovided at greater than about 1 C. In some embodiments, the initialformation charge current can be provided at greater than about 2 C. Insome embodiments, the formation charge current can be provided at lessthan or equal to about 20 C. In some embodiments, the electrochemicalcell can be a lithium-ion battery and providing the initial formationcharge current can comprise providing the formation charge current withsubstantially no lithium plating

In some instances, the first electrode can be an anode. The firstelectrode can include a silicon-dominant electrode. For example, thefirst electrode can comprise the silicon at about 30% to about 99% byweight, at about 40% to about 99% by weight, at about 50% to about 99%by weight, at about 60% to about 99% by weight, at about 70% to about99% by weight, or at about 80% to about 99% by weight. In someinstances, the first electrode can further comprise graphite.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram schematically illustrating an example method offorming an electrochemical cell in accordance with certain embodimentsdescribed herein.

FIG. 2 shows a comparison between the formation voltage profiles of twohalf-cells made with a standard graphite electrode and an examplesilicon-dominant composite electrode.

FIG. 3 is a flow diagram schematically illustrating another examplemethod of forming an electrochemical cell in accordance with certainembodiments described herein.

FIG. 4 shows the normalized discharge capacity of cells formed at 2 Cand 0.2 C charge rates.

FIG. 5 shows the initial C/5 discharge capacity to 2.75V of 3.1 Ah cellswith three different formation protocols.

FIG. 6 shows the discharge capacity retention of 3.1 Ah cells formed bythree different formation protocols.

DETAILED DESCRIPTION

Formation is a step in the production process of electrochemical cells,such as lithium-ion batteries. This step typically occurs in a factorybefore delivery of cells to a customer. The step usually involvesapplying current to the cell in such a way that causes lithium to beinserted into the negative electrode. This first “charge” can cause thesystem to undergo reversible and irreversible reactions. For stability,it may be desirable to control the reactions to control the interfaciallayer known as the solid-electrolyte interface (SEI) formed betweenelectrodes and electrolyte and to expel any gasses formed (e.g., such asin a process called degassing). In some cases, the temperature can beincreased to increase reaction rate. During formation, typically, thepotential of the negative electrode decreases. As the potential of thenegative electrode decreases, it can fall below the stability window ofthe electrolyte solution. The electrolyte solution can begin to reactwith the negative electrode, forming the SEI. In general, the SEI canreduce and/or minimize further reactions between the negative electrodeand the electrolyte solution without preventing the transport of lithiumions to and from the negative electrode. In order to control the natureof the SEI (and thus control the performance characteristics of thecell), it may be desirable to control the formation process. Typically,formation is done with low current in an environment with precisetemperature control, and this combination of long process time (due tolow current) and costly equipment (due to precise control) can makeformation a significant portion of the cost of manufacturing lithium-ionbatteries. High currents are generally avoided as they can cause lithiumplating and/or other detrimental effects. This is the case especiallywith graphite which may be the most commonly used anode active materialtoday. Accordingly, formation can be one of the most expensive parts ofmanufacturing (e.g., due to the time spent in the formation process andthe equipment for formation). Many companies are looking to reduce timein formation so that capital investment costs can be reduced.

This application describes certain methods of forming an electrochemicalcell which can be performed with decreased formation processing time andwith less sophisticated equipment, e.g., compared with currentprocesses. Some examples can include providing an electrochemical cellthat has an electrode (e.g., an anode) comprising silicon (e.g., atleast about 20% by weight of silicon). Some such electrodes can beadvantageous in that relatively high current during formation can beapplied without damage to the cell. For example, these electrodes canhave relatively high potentials such that lithium plating and otherdetrimental effects are less likely. Accordingly, some methods caninclude providing a formation charge current at greater than about 1 C(or at greater than about 1 C charge rate), which can decrease theformation time. Further, since these electrodes can handle relativelyhigh charge currents, there may be less need for precise currentcontrol. Additionally, some methods can include providing a constantcharge voltage (where current may vary) with reduced and/or minimalprogramming requirements and/or current control, which can be performedwith less costly equipment.

Formation Current

FIG. 1 is a flow diagram schematically illustrating an example method offorming an electrochemical cell in accordance with certain embodimentsdescribed herein. The method 100 of forming an electrochemical cell caninclude providing an electrochemical cell as shown in block 110. Theelectrochemical cell can include a first and second electrode, aseparator between the first and second electrodes, and electrolyte incontact with the first electrode, the second electrode, and theseparator. At least one of the electrodes can comprise at least about20% by weight of silicon (e.g., at least about 20% to about 99% byweight of silicon).

Various such electrodes as described herein can handle relatively highcharge current and/or charge rate without damage to the cell.Accordingly, as shown in block 120, the method 100 of certainembodiments can include providing a formation charge current at greaterthan about 1 C to the cell.

Without being bound by theory, one reason various embodiments ofelectrodes (e.g., anodes) described herein can take relatively highcurrents is because the electrodes have a relatively high potential, forexample, compared to graphite vs Li/Li+. Because various siliconelectrodes described herein are relatively high in voltage, theelectrodes may not be fully utilized, and can have excess capacity.

In contrast, most electrodes that are considered “silicon anodes” in theindustry are graphite anodes with silicon added in relatively smallquantities (typically <20%). These graphite-silicon mixture anodes haveto utilize the graphite which has a lower voltage compared to silicon.Because of this, the silicon has to be mostly fully utilized to utilizethe graphite. Accordingly, these graphite electrodes may not have theadvantage of various embodiments of silicon or silicon compositeelectrodes where the voltage of the electrode is substantially above 10mV vs Li/Li+.

FIG. 2 shows a comparison between the formation voltage profiles of twohalf-cells made with a standard graphite electrode and an examplesilicon-dominant composite electrode. In both cases, the electrodes werelithiated to their designed capacities. As shown in FIG. 2, thepotential of the graphite electrode fell to ˜10 mV vs Li/Li+, whereasthe potential of the silicon-dominant composite electrode stabilized at˜70 mV vs Li/Li+ after the initial activation. The graphite electrodewas a 3.1 mA/cm², 96% graphite, 4% PVdF coated anode. It was formed at0.68 mA/cm² to 10 mV and tapered to 0.11 mA/cm². The silicon-dominantcomposite electrode was a 2.97 mA/cm², 80% silicon composite anode. Itwas formed at 0.30 mA/cm² to nominal capacity. The relatively lowpotential of the graphite electrode can be problematic becausenon-uniformities in current density, diffusion, or charge transfer ratecan create localized areas of low potential where lithium plating canoccur. In contrast, the relatively high potential of thesilicon-dominate composite electrode makes lithium plating dramaticallyless likely. Via Ohm's law, the relative rate advantage of thesilicon-dominant composite electrode over the graphite electrode at agiven impedance can be expressed as the ratio of the characteristicpotentials vs. Li/Li+. Accordingly, the example silicon-dominantcomposite electrode can have a 7× improvement (e.g., 70 mV/10 mV) inrate capability over the graphite anode.

With reference to block 110 in FIG. 1, the provided electrochemical cellcan include at least one electrode comprising a silicon-dominantelectrode. The electrode can include at least about 20% by weight ofsilicon. In certain embodiments, the electrode can include at leastabout 20% to about 99% by weight of silicon. For example, the weightpercentage of silicon can be any value within this range (e.g., about20%, about 25%, about 30%, about 35%, about 40%, about 45%, about 50%,about 55%, about 60%, about 65%, about 70%, about 75%, about 80%, about85%, about 90%, about 95%, about 96%, about 97%, about 98%, about 99%,etc.), or any range within this range such as any range formed by theexample values (e.g., about 30% to about 99%, about 40% to about 99%,about 50% to about 99%, about 60% to about 99%, about 70% to about 99%,about 80% to about 99%, about 90% to about 99%, about 30% to about 95%,about 40% to about 95%, about 50% to about 95%, about 60% to about 95%,about 70% to about 95%, about 80% to about 95%, about 30% to about 90%,about 40% to about 90%, about 50% to about 90%, about 60% to about 90%,about 70% to about 90%, about 80% to about 90%, etc.). The electrode canalso include additives such as conductive material (e.g., one or moremetals or alloys) and/or graphite active material. U.S. patentapplication Ser. No. 13/008,800, filed Jan. 18, 2011, and published onJul. 21, 2011 as U.S. Patent Application Publication No. 2011/0177393,entitled “Composite Materials for Electrochemical Storage;” U.S. patentapplication Ser. No. 13/601,976, filed Aug. 31, 2012, and published onJun. 19, 2014 as U.S. Patent Application Publication No. 2014/0170498,entitled “Silicon Particles for Battery Electrodes;” and U.S. patentapplication Ser. No. 13/799,405, filed Mar. 13, 2013, and published onJun. 19, 2014 as U.S. Patent Application Publication No. 2014/0166939,entitled “Silicon Particles for Battery Electrodes,” each of which areincorporated by reference herein, describe certain embodiments ofcarbon-silicon composite materials using carbonized polymer and siliconmaterial which can be used as the silicon-dominant electrode. Furtherexample embodiments of the electrode are described herein.

The silicon-dominant electrode can be used as an anode, a cathode, orboth an anode and cathode. For example, the composite material can beused as an anode in electrochemical cells, e.g., most conventionallithium-ion batteries; they may also be used as the cathode in someelectrochemical couples, e.g., with additional additives. In someinstances, the voltage of the electrodes can be designed to be higherthan about 20 mV to about 70 mV vs Li/Li+. Various embodiments of theprovided electrochemical cells can be either secondary batteries (e.g.,rechargeable) or primary batteries (e.g., non-rechargeable). Theelectrochemical cell can have a rated capacity of 1 C.

With reference to block 120 in FIG. 1, the formation charge current canbe provided at greater than about 1 C (e.g., greater than about 1.2 C,greater than about 1.5 C, greater than about 1.7 C, etc.) to theelectrochemical cell. In some embodiments, the formation charge currentcan be provided at between about 1 C to about 20 C. For example, theformation charge current can be provided at any charge rate within thisrange (e.g., about 1 C, about 1.2 C, about 1.5 C, about 1.7 C, about 2C, about 2.5 C, about 3 C, about 3.5 C, about 4 C, about 4.5 C, about 5C, about 7 C, about 10 C, about 12 C, about 15 C, about 17 C, about 20C, etc.), or any range within this range such as any range formed by theexample values (e.g., about 1.2 C to about 20 C, about 1.5 C to about 20C, about 1.7 C to about 20 C, about 2 C to about 20 C, about 1.5 C toabout 15 C, about 2 C to about 15 C, about 1.5 C to about 10 C, about 2C to about 10 C, etc.). Other examples are possible. In some instances,the formation charge current can be substantially constant. In some suchinstances, the formation charge current can be substantially constantfor a certain period of time followed by a taper charge to a lower rate,for example, as under a constant current, constant voltage procedure. Insome cases, the formation charge current may vary.

In various embodiments, the formation charge current can be provided tothe electrochemical cell with substantially no lithium plating and/orother undesired side reactions during formation. Because thesilicon-dominant electrodes can accept relatively high charge rates,formation can be performed in a fraction of time compared toconventional manufacturing. In some embodiments, the formation chargecurrent can be provided for only about 3 minutes to about 60 minutes(e.g., compared to about 360 minutes for conventional manufacturing).For example, the amount of time the formation charge current is providedcan be any amount of time within this range (e.g., about 3 minutes,about 3.5 minutes, about 4 minutes, about 4.5 minutes, about 5 minutes,about 10 minutes, about 15 minutes, about 20 minutes, about 25 minutes,about 30 minutes, about 35 minutes, about 40 minutes, about 45 minutes,about 60 minutes, etc.), or any range within this range such as anyrange formed by the example values (e.g., about 3 minutes to about 50minutes, about 3 minutes to about 45 5 minutes, about 3 minutes to about30 minutes, about 3 minutes to about 20 minutes, about 3 minutes toabout 10 minutes, about 5 minutes to about 50 minutes, about 5 minutesto about 45 minutes, about 5 minutes to about 30 minutes, about 5minutes to about 20 minutes, about 5 minutes to about 10 minutes, etc.).The whole formation process, which may typically include additionalsteps such as rests, aging at temperature, or discharge, may becompleted in about 10 minutes to about 120 minutes (compared to about 15hours to about 60 hours for conventional manufacturing). Other examplesare possible.

In some instances, to further reduce processing time and/or reduceirreversible capacity, the cell can be charged to partial formation. Forexample, the formation charge current can be provided from about 10% toabout 90% of the total capacity of the cell. For example, the formationcharge current can be provided to any percentage within this range(e.g., about 10%, about 15%, about 20%, about 25%, about 30%, about 35%,about 40%, about 45%, about 50%, about 55%, about 60%, about 65%, about70%, about 75%, about 80%, about 85%, about 90%, etc.), or any rangewithin this range such as any range formed by the example values (e.g.,about 15% to about 90%, about 20% to about 90%, about 30% to about 90%,about 40% to about 90%, about 50% to about 90%, about 15% to about 80%,about 20% to about 80%, about 30% to about 80%, about 40% to about 80%,about 50% to about 80%, about 15% to about 70%, about 20% to about 70%,about 30% to about 70%, about 40% to about 70%, about 50% to about 70%,about 15% to about 60%, about 20% to about 60%, about 30% to about 60%,about 40% to about 60%, about 50% to about 60%, about 15% to about 50%,about 20% to about 50%, about 30% to about 50%, etc.). Other examplesare possible.

Constant Voltage

FIG. 3 is a flow diagram schematically illustrating another examplemethod of forming an electrochemical cell in accordance with certainembodiments described herein. The method 200 of forming anelectrochemical cell can include providing an electrochemical cell asshown in block 210. The electrochemical cell can include any of theelectrochemical cells as described herein with respect to the providedelectrochemical cell described in relation to FIG. 1. Because theelectrochemical cells have electrodes that can handle relatively highcharge current and/or charge rate without damage and/or undesirableeffects to the cell, there can be less of a need for precise currentcontrol. In some embodiments, formation can be performed using aconstant charge voltage (e.g., providing initial charge current at asubstantially constant charge voltage). Although current may vary undera constant charge voltage, the need for precise current control and/orother programing requirements can be reduced and/or minimized. As shownin block 220, the method 200 of certain embodiments can includeproviding a formation charge current at substantially constant chargevoltage to the cell. Accordingly, various embodiments of methods can beperformed on less sophisticated (and less costly) equipment.

With reference to block 220 in FIG. 3, the substantially constant chargevoltage can be from about 3V to about 6V. For example, the substantiallyconstant charge voltage can be any value within this range (e.g., about3V, about 3.1V, about 3.2V, about 3.3V, about 3.4V, about 3.5V, about3.6V, about 3.7V, about 3.8V, about 3.9V, about 4.0V, about 4.1V, about4.2V, about 4.3V, about 4.4V, about 4.5V, about 4.6V, about 4.7V, about4.8V, about 4.9V, about 5.0V, about 5.1V, about 5.2V, about 5.3V, about5.4V, about 5.5V, about 5.6V, about 5.7V, about 5.8V, about 5.9V, about6.0V, etc.), or any range within this range such as any range formed bythe example values (e.g., about 3.1V to about 6V, about 3.2V to about6V, about 3.3V to about 6V, about 3.4V to about 6V, about 3.5V to about6V, about 3V to about 5.5V, about 3.1V to about 5.5V, about 3.2V toabout 5.5V, about 3.3V to about 5.5V, about 3.4V to about 5.5V, about3.5V to about 5.5V, etc.). Other examples are possible.

In various embodiments, the initial formation charge current at asubstantially constant charge voltage can be provided to theelectrochemical cell with substantially no lithium plating and/or otherundesired side reactions during formation. Because the silicon-dominantelectrodes can accept relatively high charge rates, formation chargecurrent can be provided at greater than about 1 C to the cell (e.g., asdescribed herein, greater than about 1.2 C, greater than about 1.5 C,greater than about 1.7 C, greater than about 2 C, etc.). In various suchembodiments, as described herein, since the formation charge current canbe provided at greater than about 1 C, formation can be performed in afraction of time compared to conventional manufacturing. In addition, asalso described herein, to further reduce processing time and/or reduceirreversible capacity, the cell can be charged to partial formation. Forexample, as described herein, the formation charge current can beprovided from about 10% to about 90% of the total capacity.

Electrodes and Methods of Forming the Same

In various embodiments, the electrodes (e.g., anodes and cathodes) mayinclude carbonized polymer and silicon material. For example, a mixturethat includes a carbon precursor including silicon material can beformed into a composite material. This mixture can include both carbonand silicon and thus can be referred to as a carbon-silicon compositematerial, a silicon-carbon composite material, a carbon compositematerial, or a silicon composite material.

Typical carbon anode electrodes include a current collector such as acopper sheet. Carbon is deposited onto the collector along with aninactive binder material. Carbon is often used because it has excellentelectrochemical properties and is also electrically conductive. Anodeelectrodes used in the rechargeable lithium-ion cells typically have aspecific capacity of approximately 200 milliamp hours per gram(including the metal foil current collector, conductive additives, andbinder material). Graphite, the active material used in most lithium-ionbattery anodes, has a theoretical energy density of 372 milliamp hoursper gram (mAh/g). In comparison, silicon has a high theoretical capacityof 4200 mAh/g. Silicon, however, swells in excess of 300% uponlithiation. Because of this expansion, anodes including silicon mayexpand/contract and lose electrical contact to the rest of the anode.Therefore, a silicon anode should be designed to be able to expand whilemaintaining good electrical contact with the rest of the electrode.

U.S. patent application Ser. No. 13/008,800, U.S. patent applicationSer. No. 13/601,976, and U.S. patent application Ser. No. 13/799,405,each of which are incorporated by reference herein, describe certainembodiments of carbon-silicon composite materials using carbonizedpolymer and silicon material. The carbonized polymer can act as anexpansion buffer for silicon particles during cycling so that a highcycle life can be achieved. In certain embodiments, the resultingelectrode can be an electrode that is comprised substantially of activematerial. For example, the carbonized polymer can form a substantiallycontinuous conductive carbon phase(s) in the entire electrode as opposedto particulate carbon suspended in a non-conductive binder in one classof conventional lithium-ion battery electrodes. Because the polymer canbe converted into an electrically conductive and electrochemicallyactive matrix, the resulting electrode can be conductive enough that ametal foil or mesh current collector may be omitted, minimized, orreduced in some embodiments. Accordingly, in U.S. patent applicationSer. No. 13/008,800, application Ser. No. 13/601,976, and U.S. patentapplication Ser. No. 13/799,405, certain embodiments of monolithic,self-supported electrodes are disclosed. The electrodes can have a highenergy density of between about 500 mAh/g to about 3500 mAh/g that canbe due to, for example, 1) the use of silicon, 2) elimination orsubstantial reduction of metal current collectors, and 3) beingcomprised entirely or substantially entirely of active material.

The carbon-silicon composite material can be formed by forming a mixtureincluding a precursor, and pyrolyzing the precursor to convert theprecursor to a carbon phase. In certain embodiments, the carbonprecursor is a hydrocarbon compound. For example, the precursor caninclude polyamideimide, polyamic acid, polyimide, etc. Other precursorsinclude phenolic resins, epoxy resins, and other polymers. The mixturecan further include a solvent. For example, the solvent can beN-methyl-pyrollidone (NMP). Other possible solvents include acetone,diethyl ether, gamma butyrolactone, isopropanol, dimethyl carbonate,ethyl carbonate, dimethoxyethane, etc. Examples of precursor and solventsolutions include PI-2611 (HD Microsystems), PI-5878G (HD Microsystems)and VTEC PI-1388 (RBI, Inc.). PI-2611 is comprised of >60%n-methyl-2-pyrollidone and 10-30%s-biphenyldianhydride/p-phenylenediamine. PI-5878G is comprised of >60%n-methylpyrrolidone, 10-30% polyamic acid of pyromelliticdianhydride/oxydianiline, 10-30% aromatic hydrocarbon (petroleumdistillate) including 5-10% 1,2,4-trimethylbenzene. In certainembodiments, the amount of precursor (e.g., solid polymer) in thesolvent is about 10 wt. % to about 30 wt. %.

The mixture can include silicon particles as described herein. Themixture may comprise about 5% to about 80% by weight of the precursor,and at least about 20% to about 99% by weight of the silicon particles.Additional materials can also be included in the mixture. As an example,carbon particles including graphite active material, chopped or milledcarbon fiber, carbon nanofibers, carbon nanotubes, and other conductivecarbons can be added to the mixture. Conductive particles can also beadded to the mixture. In addition, the mixture can be mixed tohomogenize the mixture.

In certain embodiments, the mixture is cast on a substrate. In someembodiments, casting includes using a gap extrusion, tape casting, or ablade casting technique. The blade casting technique can includeapplying a coating to the substrate by using a flat surface (e.g.,blade) which is controlled to be a certain distance above the substrate.A liquid or slurry can be applied to the substrate, and the blade can bepassed over the liquid to spread the liquid over the substrate. Thethickness of the coating can be controlled by the gap between the bladeand the substrate since the liquid passes through the gap. As the liquidpasses through the gap, excess liquid can also be scraped off. Forexample, the mixture can be cast on a substrate comprising a polymersheet, a polymer roll, and/or foils or rolls made of glass or metal. Themixture can then be dried to remove the solvent. For example, a polyamicacid and NMP solution can be dried at about 110° C. for about 2 hours toremove the NMP solution. The dried mixture can then be removed from thesubstrate. For example, an aluminum substrate can be etched away withHCl. Alternatively, the dried mixture can be removed from the substrateby peeling or otherwise mechanically removing the dried mixture from thesubstrate. In some embodiments, the substrate comprises polyethyleneterephthalate (PET), including for example Mylar®. In certainembodiments, the dried mixture is a film or sheet. In some embodiments,the dried mixture is optionally cured. In some embodiments, the driedmixture may be further dried. For example, the dried mixture can placedin a hot press (e.g., between graphite plates in an oven). A hot presscan be used to further dry and/or cure and to keep the dried mixtureflat. For example, the dried mixture from a polyamic acid and NMPsolution can be hot pressed at about 200° C. for about 8 to 16 hours.Alternatively, the entire process including casting and drying can bedone as a roll-to-roll process using standard film-handling equipment.The dried mixture can be rinsed to remove any solvents or etchants thatmay remain. For example, de-ionized (DI) water can be used to rinse thedried mixture. In certain embodiments, tape casting techniques can beused for the casting. In some embodiments, the mixture can be coated ona substrate by a slot die coating process (e.g., metering a constant orsubstantially constant weight and/or volume through a set orsubstantially set gap). In some other embodiments, there is no substratefor casting and the anode film does not need to be removed from anysubstrate. The dried mixture may be cut or mechanically sectioned intosmaller pieces.

The mixture further goes through pyrolysis to convert the polymerprecursor to carbon. In certain embodiments, the mixture is pyrolysed ina reducing atmosphere. For example, an inert atmosphere, a vacuum and/orflowing argon, nitrogen, or helium gas can be used. In some embodiments,the mixture is heated to about 900° C. to about 1350° C. For example,polyimide formed from polyamic acid can be carbonized at about 1175° C.for about one hour. In certain embodiments, the heat up rate and/or cooldown rate of the mixture is about 10° C./min. A holder may be used tokeep the mixture in a particular geometry. The holder can be graphite,metal, etc. In certain embodiments, the mixture is held flat. After themixture is pyrolysed, tabs can be attached to the pyrolysed material toform electrical contacts. For example, nickel, copper or alloys thereofcan be used for the tabs.

In certain embodiments, one or more of the methods described herein canbe carried out in a continuous process. In certain embodiments, casting,drying, possibly curing and pyrolysis can be performed in a continuousprocess. For example, the mixture can be coated onto a glass or metalcylinder. The mixture can be dried while rotating on the cylinder tocreate a film. The film can be transferred as a roll or peeled and fedinto another machine for further processing. Extrusion and other filmmanufacturing techniques known in industry could also be utilized priorto the pyrolysis step.

Pyrolysis of the precursor results in a carbon material (e.g., at leastone carbon phase). In certain embodiments, the carbon material is a hardcarbon. In some embodiments, the precursor is any material that can bepyrolysed to form a hard carbon. When the mixture includes one or moreadditional materials or phases in addition to the carbonized precursor,a composite material can be created. In particular, the mixture caninclude silicon particles, creating a silicon-carbon (e.g., at least onefirst phase comprising silicon and at least one second phase comprisingcarbon) or silicon-carbon-carbon (e.g., at least one first phasecomprising silicon, at least one second phase comprising carbon, and atleast one third phase comprising carbon) composite material.

Silicon particles can increase the specific lithium insertion capacityof the composite material. When silicon absorbs lithium ions, itexperiences a large volume increase on the order of 300+ volume percentwhich can cause electrode structural integrity issues. In addition tovolumetric expansion related problems, silicon is not inherentlyelectrically conductive, but becomes conductive when it is alloyed withlithium (e.g., lithiation). When silicon de-lithiates, the surface ofthe silicon losses electrical conductivity. Furthermore, when siliconde-lithiates, the volume decreases which results in the possibility ofthe silicon particle losing contact with the matrix. The dramatic changein volume also results in mechanical failure of the silicon particlestructure, in turn, causing it to pulverize. Pulverization and loss ofelectrical contact have made it a challenge to use silicon as an activematerial in lithium-ion batteries. A reduction in the initial size ofthe silicon particles can prevent further pulverization of the siliconpowder as well as minimizing the loss of surface electricalconductivity. Furthermore, adding material to the composite that canelastically deform with the change in volume of the silicon particlescan reduce the chance that electrical contact to the surface of thesilicon is lost. For example, the composite material can include carbonssuch as graphite which contributes to the ability of the composite toabsorb expansion and which is also capable of intercalating lithium ionsadding to the storage capacity of the electrode (e.g., chemicallyactive). Therefore, the composite material may include one or more typesof carbon phases.

As described herein, in order to increase volumetric and gravimetricenergy density of lithium-ion batteries, silicon may be used as theactive material for the cathode or anode. Several types of siliconmaterials, e.g., silicon nanopowders, silicon nanofibers, poroussilicon, and ball-milled silicon, are viable candidates as activematerials for the negative or positive electrode. As described herein,the amount of silicon provided in the mixture or in the compositematerial can be within a range from about 20% to about 99% by weight ofthe composite material.

In some embodiments, all, substantially all, or at least some of thesilicon particles may have a particle size (e.g., the diameter or thelargest dimension of the particle) less than about 50 μm, less thanabout 40 μm, less than about 30 μm, less than about 20 μm, less thanabout 10 μm, less than about 1 μm, between about 10 nm and about 50 μm,between about 10 nm and about 40 μm, between about 10 nm and about 30μm, between about 10 nm and about 20 μm, between about 0.1 μm and about20 μm, between about 0.5 μm and about 20 μm, between about 1 μm andabout 20 μm, between about 1 μm and about 15 μm, between about 1 μm andabout 10 μm, between about 10 nm and about 10 μm, between about 10 nmand about 1 μm, less than about 500 nm, less than about 100 nm, andabout 100 nm. For example, in some embodiments, the average particlesize (or the average diameter or the average largest dimension) or themedian particle size (or the median diameter or the median largestdimension) of the silicon particles can be less than about 50 μm, lessthan about 40 μm, less than about 30 μm, less than about 20 μm, lessthan about 10 μm, less than about 1 μm, between about 10 nm and about 50μm, between about 10 nm and about 40 μm, between about 10 nm and about30 μm, between about 10 nm and about 20 μm, between about 0.1 μm andabout 20 μm, between about 0.5 μm and about 20 μm, between about 1 μmand about 20 μm, between about 1 μm and about 15 μm, between about 1 μmand about 10 μm, between about 10 nm and about 10 μm, between about 10nm and about 1 μm, less than about 500 nm, less than about 100 nm, andabout 100 nm. In some embodiments, the silicon particles may have adistribution of particle sizes. For example, at least about 95%, atleast about 90%, at least about 85%, at least about 80%, at least about70%, or at least about 60% of the particles may have the particle sizedescribed herein.

In certain embodiments, the silicon particles are at least partiallycrystalline, substantially crystalline, and/or fully crystalline.Furthermore, the silicon particles may or may not be substantially puresilicon. For example, the silicon particles may be substantially siliconor may be a silicon alloy. In one embodiment, the silicon alloy includessilicon as the primary constituent along with one or more otherelements.

Advantageously, the silicon particles described herein can improveperformance of electro-chemically active materials such as improvingcapacity and/or cycling performance. Furthermore, electro-chemicallyactive materials having such silicon particles may not significantlydegrade as a result of lithiation of the silicon particles.

The amount of carbon obtained from the precursor can be greater than 0%to about 80% by weight such as about 1% to about 80% by weight, about 1%to about 70% by weight, about 1% to about 60% by weight, about 1% toabout 50% by weight, about 1% to about 40% by weight, about 1% to about30% by weight, about 5% to about 80% by weight, about 5% to about 70% byweight, about 5% to about 60% by weight, about 5% to about 50% byweight, about 5% to about 40% by weight, about 5% to about 30% byweight, about 10% to about 80% by weight, about 10% to about 70% byweight, about 10% to about 60% by weight, about 10% to about 50% byweight, about 10% to about 40% by weight, about 10% to about 30% byweight, about 10% to about 25% by weight, etc. For example, the amountof carbon obtained from the precursor can be about 1%, about 5%, about10% by weight, about 15% by weight, about 20% by weight, about 25% byweight, etc. from the precursor.

The carbon from the precursor can be hard carbon. Hard carbon can be acarbon that does not convert into graphite even with heating in excessof 2800 degrees Celsius. Precursors that melt or flow during pyrolysisconvert into soft carbons and/or graphite with sufficient temperatureand/or pressure. Hard carbon may be selected since soft carbonprecursors may flow and soft carbons and graphite are mechanicallyweaker than hard carbons. Other possible hard carbon precursors caninclude phenolic resins, epoxy resins, and other polymers that have avery high melting point or are crosslinked. The amount of hard carbon inthe composite material can be any of the ranges described herein withrespect to the amount of carbon obtained from the precursor. Forexample, in some embodiments, the amount of hard carbon in the compositematerial can have a value within a range of greater than 0% to about80%, such as about 1% to about 10%, about 1% to about 20%, about 1% toabout 25%, about 1% to about 30%, about 1% to about 40%, about 1% toabout 50%, about 5% to about 10%, about 5% to about 20%, about 5% toabout 25%, about 5% to about 30%, about 5% to about 40%, about 5% toabout 50%, about 10% to about 25% by weight, about 10% to about 30% byweight, about 10% to about 40% by weight, about 10% to about 50% byweight, etc. In some embodiments, the amount of hard carbon in thecomposite material can be about 1% by weight, about 5% by weight, about10% by weight, about 20% by weight, about 30% by weight, about 40% byweight, about 50% by weight, or more than about 50% by weight. Incertain embodiments, the hard carbon phase is substantially amorphous.In other embodiments, the hard carbon phase is substantiallycrystalline. In further embodiments, the hard carbon phase includesamorphous and crystalline carbon. The hard carbon phase can be a matrixphase in the composite material. The hard carbon can also be embedded inthe pores of the additives including silicon. The hard carbon may reactwith some of the additives to create some materials at interfaces. Forexample, there may be a silicon carbide layer between silicon particlesand the hard carbon.

In some embodiments, graphite is one of the types of carbon phases fromthe precursor. In certain embodiments, graphite particles are added tothe mixture. Advantageously, graphite can be an electrochemically activematerial in the battery as well as an elastic deformable material thatcan respond to volume change of the silicon particles. Graphite is thepreferred active anode material for certain classes of lithium-ionbatteries currently on the market because it has a low irreversiblecapacity. Additionally, graphite is softer than hard carbon and canbetter absorb the volume expansion of silicon additives. In certainembodiments, all, substantially all, or at least some of the graphiteparticles may have a particle size (e.g., a diameter or a largestdimension) between about 0.5 microns and about 20 microns. In someembodiments, an average particle size (e.g., an average diameter or anaverage largest dimension) or median particle size (e.g., a mediandiameter or a median largest dimension) of the graphite particles isbetween about 0.5 microns and about 20 microns. In some embodiments, thegraphite particles may have a distribution of particle sizes. Forexample, at least about 95%, at least about 90%, at least about 85%, atleast about 80%, at least about 70%, or at least about 60% of theparticles may have the particle size described herein. In certainembodiments, the composite material can include graphite particles in anamount greater than 0% and less than about 80% by weight, including from40% to about 75% by weight, from about 5% to about 30% by weight, from5% to about 25% by weight, from 5% to about 20% by weight, or from 5% toabout 15% by weight.

In certain embodiments, conductive particles which may also beelectrochemically active are added to the mixture. Such particles canenable both a more electronically conductive composite as well as a moremechanically deformable composite capable of absorbing the largevolumetric change incurred during lithiation and de-lithiation. Incertain embodiments, all, substantially all, or at least some of theconductive particles can have a particle size (e.g., the diameter or thelargest dimension) between about 10 nanometers and about 7 micrometers.In some embodiments, an average particle size (e.g., an average diameteror an average largest dimension) or a median particle size (e.g., amedian diameter or a median largest dimension) of the conductiveparticles is between about 10 nm and about 7 micrometers. In someembodiments, the conductive particles may have a distribution ofparticle sizes. For example, at least about 95%, at least about 90%, atleast about 85%, at least about 80%, at least about 70%, or at leastabout 60% of the particles may have the particle size described herein.

In certain embodiments, the mixture includes conductive particles in anamount greater than zero and up to about 80% by weight. In someembodiments, the composite material includes about 45% to about 80% byweight. The conductive particles can be conductive carbon includingcarbon blacks, carbon fibers, carbon nanofibers, carbon nanotubes, etc.Many carbons that are considered as conductive additives that are notelectrochemically active become active once pyrolysed in a polymermatrix. Alternatively, the conductive particles can be metals or alloysincluding copper, nickel, or stainless steel.

After the precursor is pyrolyzed, the resulting carbon material can be aself-supporting monolithic structure. The carbonized precursor resultsin an electrochemically active structure that holds the compositematerial together. For example, the carbonized precursor can be asubstantially continuous phase. Advantageously, the carbonized precursorcan be a structural material as well as an electro-chemically active andelectrically conductive material. In certain embodiments, materialparticles added to the mixture are homogenously or substantiallyhomogeneously distributed throughout the composite material to form ahomogeneous or substantially homogeneous composite.

In some embodiments, the composite material and/or electrode does notinclude a polymer beyond trace amounts that remain after pyrolysis ofthe precursor. In further embodiments, the composite material and/orelectrode does not include a non-electrically conductive binder. Thecomposite material may also include porosity. In some embodiments, thecomposite material (or the film) can include porosity of about 1% toabout 70% or about 5% to about 50% by volume porosity. For example, theporosity can be about 5% to about 40% by volume porosity.

A current collector may be preferred in some applications, for example,where current above a certain threshold or additional mechanical supportmay be desired. U.S. patent application Ser. No. 13/333,864, filed Dec.21, 2011, and published on Jun. 19, 2014 as U.S. Patent ApplicationPublication No. 2014/0170482, entitled “Electrodes, ElectrochemicalCells, and Methods of Forming Electrodes and Electrochemical Cells,” andU.S. patent application Ser. No. 13/796,922, filed Mar. 12, 2013, andpublished on Jun. 19, 2014 as U.S. Patent Application Publication No.2014/0170475, entitled “Electrodes, Electrochemical Cells, and Methodsof Forming Electrodes and Electrochemical Cells,” each of which isincorporated by reference herein, describe certain embodiments ofelectrodes including a current collector, electrochemical cellscomprising such electrodes, and methods of forming such electrodes andelectrochemical cells.

In some embodiments, the full capacity of the composite material of theelectrodes described herein may not be utilized during use of thebattery to improve life of the battery (e.g., number charge anddischarge cycles before the battery fails or the performance of thebattery decreases below a usability level). For example, a compositematerial with about 70% by weight of silicon particles, about 20% byweight of carbon from a precursor, and about 10% by weight of graphitemay have a maximum gravimetric capacity of about 3000 mAh/g, while thecomposite material may only be used up to an gravimetric capacity ofabout 550 to about 1500 mAh/g. Although, the maximum gravimetriccapacity of the composite material may not be utilized, using thecomposite material at a lower capacity can still achieve a highercapacity than certain lithium ion batteries. In certain embodiments, thecomposite material is used or only used at an gravimetric capacity belowabout 70% of the composite material's maximum gravimetric capacity. Forexample, the composite material is not used at an gravimetric capacityabove about 70% of the composite material's maximum gravimetriccapacity. In further embodiments, the composite material is used or onlyused at an gravimetric capacity below about 50% of the compositematerial's maximum gravimetric capacity or below about 30% of thecomposite material's maximum gravimetric capacity.

EXAMPLES

The following examples are provided to demonstrate the benefits of someembodiments of electrodes, electrochemical cells, and methods of formingthe same. These examples are discussed for illustrative purposes andshould not be construed to limit the scope of the disclosed embodiments.

FIG. 4 shows the normalized discharge capacity retention of cells formedat 2 C and 0.2 C charge rates. The cells contained 90% lithium nickelcobalt aluminum oxide (NCA) cathodes and 70% silicon-dominant compositeanodes in accordance with certain embodiments described herein. Thefirst formation charge was at 2 C charge rate or 0.2 C charge rate. Thefirst charge was to 4.2V with a taper charge to C/20, followed by 5hours rest, and 0.2 C discharge to 3.1V. The cells' performances wereevaluated by cycling them between 4.2 to 3.1V at 1 C charge (with taperto C/20) and 1 C discharge rates. As shown in FIG. 4, the data shows nosignificant difference between charging at 2 C vs 0.2 C for the constantcurrent portion in a constant current/constant voltage regime.

In another example, cells were constructed using silicon-dominantcomposite anodes with 80% silicon content by weight, with averageloading of 3.8 mg/cm² and average thickness of 30 μm, attached to 15 μmcopper foil with polyamideimide adhesive. The cathodes were 97% lithiumcobalt oxide, 1% conductive additive, and 2% PVdF, coated on 15 μmaluminum foil at an average loading of 28 mg/cm² and an average densityof 4.0 g/cc. The electrolyte was a solution of 1.2M LiPF₆ in organiccarbonates. The cell design included 19 layers, with overall dimensionsof 38.5 mm×64.0 mm×6.0 mm and a nominal capacity of 3.1 Ah. The cellswere formed using 3 different formation protocols (2 cells each):

A. 1 C charge to 4.3V, Constant Voltage at 4.3V to C/20.

B. Constant Voltage at 4.3V to C/20.

C. Constant Voltage at 4.3V for 10 minutes.

FIG. 5 shows the initial C/5 discharge capacity to 2.75V of the 3.1 Ahcells with the three different formation protocols. FIG. 6 shows thedischarge capacity retention of the cells. The cells' performancecharacteristics were evaluated using a protocol in which they werecharged at 2 C to 4.3V, held at constant voltage to C/20, and dischargedat 0.5 C to 3.3V (discharged to 2.75V every 50 cycles). The cells hadvery similar capacity retention on a normalized basis, but the cellswith formation B had lower initial discharge capacity. Reducing theconstant voltage formation from a full charge (formation B) to a partialcharge (formation C) resulted in an increase in capacity to match thatof the 1 C formation (formation A), with the added benefit of reducingthe charge time from 210 minutes (compared to Formation A) and from 99minutes to 10 minutes (compared to Formation B).

For example, the Formation C cells completed formation charge in 10minutes, and the whole formation sequence (consisting of a rest step, acharge step, and a discharge step) was completed in 23 minutes. This wasachieved by placing a 3.1 Ah cell on a 1-minute rest to check thestability of the electrical connection, followed by a 4.3V constantvoltage taper charge for 10 minutes. The cell took 6 A immediately,which was the maximum current available from the channel, reaching ˜30%SOC in 10 minutes. The cell was then discharged at 0.7 C to 3.3V, a12-minute discharge step. The cell was cycled between 4.3V and 3.3V at 2C charge rate and 0.5 C discharge rate. Compared to a cell that wentthrough a constant current, constant voltage formation protocol(Formation A), the cell (Formation C) demonstrated similar performance.In the specific case, the formation charge step was completed in ˜5% ofthe time for Formation A (10 minutes vs. 210 minutes) and in ˜10% of thetime for Formation B (10 minutes vs. 99 minutes).

Various embodiments have been described above. Although the inventionhas been described with reference to these specific embodiments, thedescriptions are intended to be illustrative and are not intended to belimiting. Various modifications and applications may occur to thoseskilled in the art without departing from the true spirit and scope ofthe invention as defined in the appended claims.

What is claimed is:
 1. A method of forming an electrochemical cell, themethod comprising: providing an electrochemical cell comprising: a firstelectrode and a second electrode, wherein at least the first electrodecomprises at least about 20% to about 99% by weight of silicon; aseparator between the first electrode and the second electrode; and anelectrolyte in contact with the first electrode, the second electrode,and the separator; and providing a formation process comprising a chargestep, a rest step, and a discharge step, the charge step comprisingproviding a formation charge current at greater than about 1 C to theelectrochemical cell, wherein the formation process is completed inapproximately 10 minutes to 120 minutes.
 2. The method of claim 1,wherein the formation charge current is provided at less than or equalto about 20 C.
 3. The method of claim 1, wherein the formation chargecurrent is provided at greater than about 2 C.
 4. The method of claim 3,wherein the formation charge current is provided at less than or equalto about 20 C.
 5. The method of claim 1, wherein the formation chargecurrent is provided for about 3 minutes to about 60 minutes.
 6. Themethod of claim 1, wherein providing the formation charge currentcomprises charging to partial formation.
 7. The method of claim 6,wherein charging to partial formation comprises charging to about 10% toabout 90% of total capacity of the electrochemical cell.
 8. The methodof claim 7, wherein charging to partial formation comprises charging toabout 20% to about 70% of the total capacity of the electrochemicalcell.
 9. The method of claim 8, wherein charging to partial formationcomprises charging to about 30% to about 50% of the total capacity ofthe electrochemical cell.
 10. The method of claim 1, wherein providingthe formation charge current comprises providing a substantiallyconstant formation charge current.
 11. The method of claim 1, whereinproviding the formation charge current comprises providing the formationcharge current at a charge voltage from about 3 volts to about 6 volts.12. The method of claim 1, wherein the electrochemical cell is alithium-ion battery and providing the formation charge current comprisesproviding the formation charge current with substantially no lithiumplating.
 13. The method of claim 1, wherein the first electrode is ananode.
 14. The method of claim 1, wherein the first electrode comprisesa silicon-dominant electrode.
 15. The method of claim 1, wherein thefirst electrode comprises the silicon at about 30% to about 99% byweight.
 16. The method of claim 15, wherein the first electrodecomprises the silicon at about 40% to about 99% by weight.
 17. Themethod of claim 16, wherein the first electrode comprises the silicon atabout 50% to about 99% by weight.
 18. The method of claim 17, whereinthe first electrode comprises the silicon at about 60% to about 99% byweight.
 19. The method of claim 18, wherein the first electrodecomprises the silicon at about 70% to about 99% by weight.
 20. Themethod of claim 19, wherein the first electrode comprises the silicon atabout 80% to about 99% by weight.
 21. The method of claim 1, wherein thefirst electrode further comprises graphite.